The present invention relates to a displacement sensor, in particular, relates to such a device which senses the displacement or vibration of a living body. The present invention is applicable to a stethoscope for medical and mechanocardiogram purposes.
Conventionally, an acoustic stethoscope has been used for diagnosing the internal conditions of a living body. A prior stethoscope places a diaphram on a surface of a living body, and the vibration of the diaphram is listened to by a doctor through a stethoscope tube.
Therefore, a prior stethoscope has the disadvantage that only a single doctor can listen to the sound of the vibration of the heart valves of a living body, but it cannot be listened to by a plurality of persons. Further, a prior stethoscope has the disadvantage that a doctor cannot listen to the sound of a heart beat with a frequency less than 20 Hz, and the output of the stethoscope can not be recorded.
On the other hand, a prior electrocardiogram records pulsation or drive electrical signal of a heart, but it does not record the actual mechanical movement of a heart. Therefore, if there is something wrong with a heart, such as valve disease and arteriosclerosis, it could not be detected by a prior electrocardiogram. Conventional mechanocardiograph sensors for detection of heart movement have disadvantage due to their difficulty of usage.
The present applicant proposed an improved displacement sensor as shown in FIG. 1, which solves the above problem, in U.S. patent application Ser. No. 931,699 now U.S. Pat. No. 4,765,321, issued 08/23/88 and EP Patent application No. 86308971.0. The displacement sensor of FIG. 1 uses the principle of inductance control by a movable permanent magnet.
In the figure, the displacement sensor or the stethoscope 101 has a main body 102, a diaphram 103 and a sensor 110.
A hollow rigid tube 108 is coupled with the main body 102, and a flexible tube 104 is coupled with said hollow rigid tube 108. Accordingly, an acoustic vibration in empty space 105 in the main body 102 propagates through the rigid tube 108 to the flexible tube 104, far end of which is to be inserted into the ear of a doctor. The main body 102 has a screw 109 at the outer surface of the same, and the diaphram 103 is engaged with that screw. The sensor 110 is mounted between the diaphram 103 and the main body 102.
The sensor 110 functions to generate an electrical signal according to the vibration of the diaphram 103, and said sensor 110 has a magnetic pole M (a small permanent magnet), a group of inductors 112 which cause inductance variation according to displacement of said magnetic pole M, and a holder 111 for fixing the inductors 112. The sensor terminal 105a is mounted on the main body 102 so that the lead wires of said inductors 112 are coupled with the sensor cable 107 through the sensor terminal 105a. The far end of the sensor cable 107 is coupled with a processor 106 which processes the signal from the sensor 110.
Accordingly, the stethoscope of FIG. 1 functions both for a prior stethoscope which propagates acoustic vibration to an ear, and for an electric stethoscope which provides electrical output information according to the vibration of the diaphram 103.
However, we realized some problems of the stethoscope of FIG. 1. First, some noise is induced on a cable 107, because the processor circuit 106 is separated from the stethoscope 101. Furthermore, we realized that it does not need to double as an acoustic stethoscope. More importantly, to put the stethoscope accurately on an affected part of a living body for a long time is difficult. Also, since the stethoscope of FIG. 1 is applied on an affected part with a doctor's hand, the electrical output depends upon the pressure put by the stethoscope on an affected part.
In order to solve the above problems, we made a first step a displacement sensor as shown in FIG. 2, which uses the same electrical principle as that of FIG. 1, but a processor circuit is included in a stethoscope head, an acoustic stethoscope is removed, and a stethoscope is adhered to an affected part of a living body instead of applying the same by hand.
In FIG. 2, a permanent magnet 3 is put on an affected part 2 of a living body 1 by an adhesive thin film 4. The sensor body 6 has a recess 8 at the bottom of the housing 7. The sensor body 6 is put on the affected part so that the permanent magnet 3 is within the recess 8. The non-magnetic conductive shield case 9 is inserted in the housing 7, and an even number of inductors 12 each having a magnetic wire 10 and a coil 11 wound on the wire 10 are arranged radially with an equal angular interval. The inductors are molded on the inductor shelf 4 by plastics 13. The printed circuit board 16 which mounts a processor circuit 15 is positioned on the inductor shelf 14 with some spacing. A lead wire connects the process circuit 15 with an external circuit.
However, we found in the experiment that the structure of FIG. 2 has the disadvantage that it is difficult to locate the sensor body 6 so that the center of the recess 8 coincides with the center of the permanent magnet 3. It should be noted that the diameter of the magnet is about 4 mm, and the inner diameter of the recess 8 is about 10 mm so that the magnet 4 can vibrate freely in vertical direction. The locational error of the sensor body 6 causes the error of an output signal of the displacement sensor, and/or deteriorates the essential sensitivity of the sensor.